Sample site coordinates, environmental data, number of copies of target DNA/ul for each sample and limit of detection plot

Human activities in coastal areas are accelerating ecosystem changes at an
unprecedented pace, resulting in habitat loss, hydrological modifications,
and predatory species declines. Understanding how these changes
potentially cascade across marine and freshwater ecosystems requires
knowing how mobile euryhaline species link these seemingly-disparate
systems. As upper trophic level predators, bull sharks (Carcharhinus
leucas) play a crucial role in marine and freshwater ecosystem health.
Telemetry studies in Mobile Bay, Alabama suggest that bull sharks
extensively use the northern portions of the bay, an estuarine-freshwater
interface known as the Mobile-Tensaw Delta. To assess whether bull sharks
use freshwater habitats in this region, environmental DNA surveys were
conducted during the dry summer and wet winter seasons in 2018. In each
season, 5 x 1 L water samples were collected at each of 21 sites:
five sites in Mobile Bay, six sites in the Mobile-Tensaw Delta, and ten
sites throughout the Mobile-Tombigbee and Tensaw-Alabama Rivers. Water
samples were vacuum-filtered, DNA extractions were performed on the
particulate, and DNA extracts were analyzed with Droplet Digital™
Polymerase Chain Reaction using species-specific primers and an internal
probe to amplify a 237-base pair fragment of the mitochondrial NADH
dehydrogenase subunit 2 gene in bull sharks. One water sample collected
during the summer in the Alabama River met the criteria for a positive
detection, thereby confirming the presence of bull shark DNA. While
preliminary, this finding suggests that bull sharks use less urbanized,
riverine habitats up to 120 km upriver during Alabama’s dry summer season.